1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus using the optical scanning apparatus. In particular, the present invention relates to an optical scanning apparatus, which is suited for an image forming apparatus such as a laser beam printer (LBP), a digital copying machine, or a multifunctional printer (versatile printer) each including an electrophotographic process. In the optical scanning apparatus, a light beam emitted from light source means is reflected and deflected by a polygon mirror serving as a light deflector and passes through an imaging optical system, then a surface to be scanned is scanned with the light beam to record image information.
2. Related Background Art
Conventionally, in an image forming apparatus such as a laser beam printer or a digital copying machine, a light flux (beam) light-modulated in accordance with an image signal and emitted from light source means composed of a semiconductor laser or the like is periodically deflected by a light deflector composed of a rotating polygon mirror (polygon mirror) or the like. The light beam is then converged in a spot manner onto a photosensitive recording medium (photosensitive drum) surface by an imaging optical system (scanning lens system) having fθ characteristics, so that the recording medium surface is scanned with the light beam, thereby performing image recording.
FIG. 7 is a main portion sectional view (main scanning sectional view) in a main scanning direction of an optical scanning apparatus used in a conventional image forming apparatus of this type.
In the drawing, a parallel light beam emitted from a laser unit 91 including a semiconductor laser is made incident on a cylindrical lens (condensing lens) 92 that has predetermined refractive power only in a sub-scanning direction. The parallel light beam made incident on the cylindrical lens 92 is emitted as maintaining its parallel light beam state in a main scanning section.
On the other hand, the parallel light beam is converged in a sub-scanning section, and imaged as a line image elongated in the main scanning direction in proximity to a deflecting surface (reflecting surface) 93a of a light deflector 93 composed of a rotating polygon mirror. Then, the light beam reflected and deflected by the deflecting surface 93a of the light deflector 93 is imaged by an imaging optical system (fθ lens system) 94 having fθ characteristics as a light spot on a surface of a photosensitive drum 95, that is a surface to be scanned. Then, the surface of the photosensitive drum 95 is repeatedly scanned with the light spot. The imaging optical system 94 includes a spherical lens 94a and a toric lens 94b. 
In the optical scanning apparatus, a beam detector (BD) sensor 98 serving as a photodetector is provided in order to adjust a timing of start of image formation on the surface of the photosensitive drum 95 prior to scanning of the surface of the photosensitive drum 95 with the light spot. The BD sensor 98 receives a BD light beam that is a part of the light beam reflected and deflected by the light deflector 93. In other words, the BD sensor 98 receives a light beam during scanning of a region other than an image forming region prior to scanning of the image forming region on the surface of the photosensitive drum 95. The BD light beam is reflected by a BD mirror 96 and then condensed by a BD lens (condensing lens) 97 to be incident on the BD sensor 98. Then, a BD signal (synchronizing signal) is detected from an output signal of the BD sensor 98 to adjust a start timing of image recording on the surface of the photosensitive drum 95 based on the BD signal.
The photosensitive drum 95 rotates at a constant speed in synchronization with a driving signal of the semiconductor laser in the laser unit 91, and the surface of the photosensitive drum 95 moves in the sub-scanning direction with respect to the light spot with which the surface is scanned. As a result, an electrostatic latent image is formed on the surface of the photosensitive drum 95. Then, the electrostatic latent image is developed by a known electrophotographic process and transferred onto a transfer target material such as paper, whereby the electrostatic latent image is visualized.
Also, a multiple image forming apparatus using an imaging optical system generally performs image formation by forming images in different colors in a plurality of image forming portions, conveying paper using a conveyance means such as a conveyance belt, and transferring the images onto the paper to be superimposed one another.
In particular, when a full-color image is to be obtained by performing multicolor development, even a slight misregistration leads to degradation of image quality. In the case of 400 dpi, for instance, even misregistration of a fraction of one pixel (one pixel corresponds to 63.5 μm) results in a change appeared as color misregistration or color tint drift, and significantly degrades image quality. Conventionally, in view of this problem, the image drift is alleviated by performing color development using the same imaging optical system, that is, by performing light-scanning with the same optical characteristics.
With this method, however, there has been a problem in that it takes a long time to output a multiplex image or a full-color image. In order to solve the problem, there has been a method with which images in respective colors are obtained through image formation using multiple different optical scanning apparatuses, and transferred onto paper conveyed by a conveyance portion to be superimposed one another.
In this case, however, there is apprehension that color misregistration will occur when the images are superimposed one another. As an effective method of solving the problem, there has been a method with which an image position is detected and an image forming portion is controlled so as to correct an image in accordance with a detection signal (see Japanese Patent publication No. H01-281468).
Meanwhile, in an image forming apparatus in which multiple photosensitive members are scanned with light beams, imaging optical systems are ordinarily provided as many as the photosensitive members in order to form latent images on the multiple photosensitive members. In this case, there has been a problem in that optical components are required as many as the imaging optical systems, which increases cost because the light deflector (polygon mirror) and the like in particular are expensive. Also, in the case of particularly high-speed and high-definition imaging optical systems, the problem becomes more serious because the light deflectors are increased in size and required to have capacities for high-speed deflection at the same time.
In addition, a full-color image forming apparatus that is compact, inexpensive, and capable of realizing high image quality has been desired recently. As one method of satisfying this demand, there has been proposed a system in which a single common polygon mirror is used to scan with multiple light beams so that the number of components can be reduced, thereby achieving cost reduction.
In the case where a common polygon mirror is used, optical path separation is required in order to guide respective multiple beams (light beams) to different surfaces to be scanned. Therefore, a method has been proposed with which the beams are made incident on the polygon mirror at different angles in the sub-scanning direction (see Japanese Patent Application Laid-Open No. 2002-148546 and Japanese Patent Application Laid-Open No. 2004-78089).
With the conventional method, however, light beams other than a light beam having a small incident angle in the sub-scanning direction need to be made incident onto the polygon mirror at larger angles, which tends to increase the beam incident angles onto the polygon mirror in the sub-scanning section. In particular, when the imaging optical systems is a reduction system in the sub-scanning direction, the incident system tends to be increased in length in order to secure a light amount. In order to decrease the imaging optical system in size, the light beam is turned back using a turn back mirror or the like.
Also, in an overfilled imaging optical system (OFS), in order to suppress pupil diameter fluctuations due to inclination of a polygon facet, it is desirable for a scanning light beam to be so-called confrontational incident (frontal incident), so that a reflection angle of a scanning light beam on a deflecting surface at a scanning center in the main scanning section is set to be zero.
FIG. 8 is a sub-scanning sectional view of a main portion of a conventional optical scanning apparatus using a common polygon mirror.
In the drawing, reference numeral 19 denotes an incident system turn back mirror, numeral 17 a scanning lens system, numeral 18 a polygon mirror, numeral 18A a deflecting surface (reflecting surface), numeral 18B a rotation axis, and 16A and 16B each a scanning system turn back mirror. Light beams A and B are incident on the polygon mirror 18 at different incident angles (oblique incident angles), for example, 1.5° and 2.4°, in the sub-scanning section, deflected (reflected and deflected) at different angles by the deflecting surface 18A of the polygon mirror, and separated by the scanning system turn back mirrors 16A and 16B to be reflected toward different surfaces to be scanned. Note that, it is required to give an incident angle of around 1.5° to the light beam B having a smaller oblique incident angle in order to prevent the light beam B from interfering with the incident system turn back mirror 19.
In addition, in order to separate the optical paths of the scanning light beams from each other, it is required to make the respective light beams incident on the polygon mirror 18 at different angles and to make the light beam A incident on the deflecting surface 18A with a larger angle of around 2.4°.
In a case where a light beam is obliquely made incident on a polygon mirror with a large angle in a sub-scanning section, there has been a problem in that a position of the deflecting surface of the polygon mirror moves back and forth, which causes so-called pitch unevenness in which a beam reaching position in the sub-scanning direction is displaced.
With a conventional technique, the pitch unevenness is suppressed by reducing a relative amount of eccentricity of each deflecting surface of the polygon mirror. In this case, however, the cost is increased. Also, when it is impossible to adopt a high-precision polygon mirror, image quality is degraded. In addition, if the oblique incident angle is large, there have been such problems in that bending of a scanning line is easy to occur, and in that spot performance is deteriorated.